In general, this invention relates to an automatic focusing system having an objective spaced an adjustable distance from an image plane. More particularly, it relates to such a system which employs a pair of variable frequency oscillators.
Focusing of an optical system involves adjusting the spacing between an objective and an image plane to a relative position where maximum sharpness of the image is achieved. Various types of manual focusing systems are known. With the type of manual system typically used in a single lens reflex camera, a viewfinder enables viewing of the image formed on a focusing screen by mirror-reflected light that traverses the same distance from objective to focusing screen as the distance from objective to film plane. By manually rotating a focusing ring, the objective is moved to a point where the image is sharpest.
In recent years, there has been considerable activity directed to developing automatic focusing systems. It is now well known to employ servomechanism techniques for automatic focusing wherein an electric signal is produced and provided to a motor for moving the objective relative to the image plane. Various methods have been proposed as to how such an electrical signal can be produced for appropriately controlling the motor to move the objective in the proper direction and by the proper amount.
Photoconductor elements such as cadmium sulfide (CdS) cells have been employed in these methods. A CdS cell is characterized by the following phenomenon. It functions as a variable resistor having a peak resistance value when light which impinges on it is in sharp focus.
With this phenomenon in mind, consider the effect of positioning a CdS cell at an image plane spaced an adjustable distance from an objective. When the spacing between objective and image plane is such that the image formed at the image plane is in focus, the internal resistance of the CdS cell is at a maximum. If the spacing is changed in either direction, the internal resistance decreases. The functional relationship between the internal resistance and this spacing thus has a positive slope on one side of the maximum, zero slope at the maximum, and a negative slope on the opposite side of the maximum.
Significant problems have been encountered in efforts to use a CdS cell in an automatic focusing system. A principal factor underlying these problems is that this change in slope is very gradual. In other words, there is only a small rate of change of internal resistance with respect to change of spacing in the vicinity of an in-focus position under the circumstance that the CdS cell is positioned at the image plane. This factor makes it difficult to employ the CdS cell as a transducer in a conventional analog circuit arrangement for producing a control signal for the motor of the automatic focusing system.